The next section (45 rpm drive) is slightly narrower. It steps down in dia by about 4 mm from the lower section (90 deg edge on the step). This section is a "parallel cylinder", no "high or low spots"

well you might have answered that there. See the homemade pulley shown in the pic below. See the way its fatter in the middle and thinnner top and bottom - the motor angle is meant to be adjusted so that the belt rides on the widest part. But you seem to have that sorted.

Last thing I can think of:

Could you measure (accurately with a vernier callipers) the diameters of the drive pulley and the platter (where the belt rides) and work out the ratio of one to the other. So if the motor runs at 250rpm at 50Hz, the ratio of the pulleys should give you 250/33.3=7.507

Hi,
I bought a replacement belt on ebay for my BL-10X and the speed was innacurate!
So i bought another one from another shop and it did the trick!
Check your belt thickness and stifness and use only one that's exactly the sma than the genuine part!